The present invention relates to a liquid crystal display device using liquid crystal of chiral smectic C phase or its sub-phase such as ferroelectric liquid crystal or anti-ferroelectric liquid crystal as a liquid crystal material.
In order to improve a response speed and a viewing angle of TFT-LCD, various kinds of systems using the ferroelectric liquid crystal or anti-ferroelectric liquid crystal as its liquid crystal material have been studied. It is known that if the liquid crystal having their spontaneous polarization (or, more generally, the liquid crystal of chiral smectic C phase or its sub-phase) is driven by TFT, the depolarization field causes a holding voltage to be lowered when the response time of the liquid crystal is longer than the write-in time (for example, Hartmann: J. Appl. Phys. 66,1132(1989)). This lowering of the holding voltage is so called the shortage of writing, causing an effective applied voltage and a contrast ratio to be lower, which are serious problems in image quality.
Further, another problem occurs when the applied voltage is polarity-inverted in every frame and the driving (AC-driving) is carried out in positive/negative symmetrical modes. That is, when an absolute value of the signal voltage is varied at a certain frame or, in other words, when displayed images are changed, a phenomenon called "step response" that a steady state quantity of transmitted light is set after repeating bright state and dark state in a few frames, occurs (for example, Verhulst et al.: IDRC '94 digest, 377 (1994)).
When not a system of the symmetrical mode (AC driving), but a system of the asymmetrical mode (DC driving) (for example, Tanaka et al.: SID '94 digest, 430 (1994)) is employed, the "step response" does not occur and the contrast ratio is enhanced. However, the response becomes cumulative and the response speed becomes lower. This is also a phenomenon occurring as a result of the lowering of the holding voltage. The response speed becomes lower as the write-in time is shorter. Further, in the DC driving, there are problems such as image sticking caused by impurities and the afterimage caused by residual hysteresis.
As mentioned above, in both the symmetrical mode AC driving and the asymmetrical mode DC driving, the most important object for practice is to solve the two problems of the lowering of the holding voltage and the "step response".
From the viewpoint of the properties of the liquid crystal material, two measures to be taken, i.e. acceleration of the response speed and the lowering of the spontaneous polarization, are considered. These two problems can be solved by using a liquid crystal material whose response speed is sufficiently higher than that at the low voltage driving or at a temperature slightly lower than the room temperature and whose response time is shorter than the write-in time, but now there is no liquid crystal material satisfying these conditions. It is considered questionable whether the acceleration at a low temperature can be realized or not in future.
Further, the liquid crystal display device is required to be further enlarged in screen size and highly miniaturized in pixel size. For this purpose, however, the write-in time for one line inevitably needs to be shortened. Therefore, it is difficult to solve the above two problems within the limit of the response characteristic of the liquid crystal material.
Moreover, restricting the lowering of the holding voltage by reduction of the spontaneous polarization has been expected. However, the reduction of the spontaneous polarization theoretically brings about the lowering of the response speed and, as a result, the above two problems cannot be thereby solved. As described above, the only improvement of the properties of the liquid crystal material is inadequate as the measure to be taken to solve the two problems, i.e. the lowering of the holding voltage and the "step response".
Next, the measures which can be conceived from improvement of the driving method and the circuit structure will be considered. First, increase of the storage capacitance is considered. In an active matrix liquid crystal display element using general twisted nematic liquid crystal, the storage capacitance is substantially the same as the capacitance of pixels filled with the liquid crystal. By increasing the storage capacitance at ten or more times, the lowering of the holding voltage can be solved.
However, the "step response" cannot be solved unless the response speed of the liquid crystal material is substantially as low as the speed obtained at present. The quantity of current is also increased in accordance with the increase of the storage capacitance, which causes the power consumption to be increased and load on the driving circuit to be increased. Therefore, this measure is not considered applicable in practice, and the use is limited.
Further, as the measure to solve the "step response", a method of executing a resetting operation of writing voltage of approximately 0V immediately before the write and erasing or offsetting the preliminarily held charge, has been known. Driving methods using TFT or TFD have been announced, but part of the write-in time is assigned to the resetting operation in the driving methods. Therefore, the "step response" can be thereby solved. However, since the substantial write-in time is shortened unless the number of lines is reduced, the contrast is not adequately improved. In a case where the write-in time is shortened in accordance with high resolution, the write-in time is further shortened by the resetting operation, and shortage of the write becomes serious. If a circuit structure having two TFDs and two signal lines for each pixel is employed (for example, Verhulst et al.: IDRC '94 digest, 377 (1994)), the resetting operation can be executed during the write in other lines. In this structure, however, the number of the switching elements and lines for the respective pixels is increased, the driving waveform is complicated, and problems arise in relation to yield in the manufacture or manufacturing costs. In addition, a problem arises that with the TFD, irregularity in the properties of all the switching elements cannot easily be corrected and, therefore, this measure is not considered applicable in practice.